Conventionally, there is known an electron tube such as a photomultiplier tube which detects weak light such as fluorescence generated from a sample. The electron tube includes a vacuum container with a light transmitting substrate and a photocathode provided on a vacuum side inner surface of the light transmitting substrate. Non Patent Literature 1 (W. D. Gunter, Jr., G. R. Grant, and S. A. Shaw. Optical devices to increase photocathode quantum efficiency. APPLIED OPTICS Vol. 9, No. 2 251-257 (1970)) discloses a configuration in which a prism is provided in an outer surface of a light transmitting substrate provided with such a photocathode. In this configuration, light incident to an incident surface of the prism is totally reflected at an interface between the photocathode and a vacuum space and then is further reflected at a surface on the side opposite to the incident surface of the prism so as to return to the photocathode. Accordingly, the quantum efficiency (QE) of the photocathode is improved.